Notices warning employees of the expected closing will go out to the casino’s 1,000-plus employees Monday.

If Trump Plaza closes, Atlantic City could lose a third of its casinos and a quarter of its casino workforce in less than nine months. The Atlantic Club closed in January, the Showboat is closing next month, and Revel might do likewise if a buyer can’t be found in bankruptcy court.

The head of Atlantic City’s main casino workers’ union demanded state lawmakers help head off what he called a ‘‘pending catastrophe’’ that will affect the state’s tourism industry and tax collections.

Trump Entertainment Resorts said its managers and board of directors ‘‘have been reviewing alternatives for the property. Although this review has not been completed and no final decision has been made, the company expects that it will terminate the operations of Trump Plaza Hotel and Casino on or shortly after September 16, 2014.’’

A source with direct knowledge of the situation who was not authorized to speak to the media said the company has hired a search firm to solicit buyers for Trump Plaza, an effort that remains ongoing. So far, no buyer has emerged.

The company did not indicate what might become of the building after it is closed.

Bob McDevitt, president of Local 54 of the Unite-HERE union, said 7,000 casino workers — or about one in four — have been warned their jobs could disappear within 60 days.

‘‘While this is a personal tragedy for every family involved, it is also a crisis for the state,’’ he said. ‘‘We expect Trenton to react with more than just sympathetic sound bites; we demand action equal to the magnitude of this pending catastrophe.’’

Trump Plaza, which cost $210 million to build, opened in May 1984 as one of Donald Trump’s pet projects. The real estate mogul has since limited his dealings in Atlantic City to a 10 percent stake in Trump Entertainment Resorts and does not control its day-to-day operations.

The news is the latest in a cascade of setbacks for Atlantic City’s gambling market, which until just a few years ago was the second-largest in the nation after Nevada; Pennsylvania has now taken over that spot.

Analysts have long said that the casino market here, and in the Northeastern United States, has been oversaturated, and that some casinos need to close to ensure the survival of others.

On Jan. 1, Atlantic City had 12 casinos. By the end of September, it could have eight.